1. Field of the Invention
The present invention relates to transmitting data over existing cable television plants using cable modems. More specifically, it relates to reducing noise outside a particular channel created by cable modems while transmitting data on the upstream path in the cable television plant.
2. Discussion of Related Art
The cable TV industry has been upgrading its signal distribution and transmission infrastructure since the late 1980s. In many cable television markets, the infrastructure and topology of cable systems now include fiber optics as part of their signal transmission components. This has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cablecommuting.
Originally, cable TV lines were exclusively coaxial cable. The system included a cable head end, i.e. a distribution hub, which received analog signals for broadcast from various sources such as satellites, broadcast transmissions, or local TV studios. Coaxial cable from the head end was connected to multiple distribution nodes, each of which could supply many houses or subscribers. From the distribution nodes, trunk lines (linear sections of coaxial cable) extended toward remote sites on the cable network. A typical trunk line is about 10 kilometers. Branching off of these truck lines were distribution or feeder cables (40% of the system's cable footage) to specific neighborhoods, and drop cables (45% of the system's cable footage) to homes receiving cable television. Amplifiers were provided to maintain signal strength at various locations along the line. For example, broadband amplifiers are required about every 2000 feet depending on the bandwidth of the system. The maximum number of amplifiers that can be placed in a run or cascade is limited by the build-up of noise and distortion. This configuration, known as tree and branch, is still present in older segments of the cable TV market.
With cable television, a TV analog signal received at the head end of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 60 to 700 MHz. Broadcast signals were sent downstream; that is, from the head end of the cable system across the distribution nodes, over the trunk line, to feeder lines that led to the subscribers. However, the cable system did not have installed the equipment necessary for sending signals from subscribers to the head end, known as return or upstream signal transmission. Not surprisingly, nor were there provisions for digital signal transmission either downstream or upstream.
In the 1980s, cable companies began installing optical fibers between the head end of the cable system and distribution nodes (discussed in greater detail with respect to FIG. 1 below). The optical fibers reduced noise, improved speed and bandwidth, and reduced the need for amplification of signals along the cable lines. In many locations, cable companies installed optical fibers for both downstream and upstream signals. The resulting systems are known as hybrid fiber-coaxial (HFC) systems. Upstream signal transmission was made possible through the use of duplex or two-way filters. These filters allow signals of certain frequencies to go in one direction and of other frequencies to go in the opposite direction. This new upstream data transmission capability allowed cable companies to use set-top cable boxes and allowed subscribers pay-per-view functionality, i.e. a service allowing subscribers to send a signal to the cable system indicating that they want to see a certain program.
In addition, cable companies began installing fiber optic lines into the trunk lines of the cable system in the late 1980s. A typical fiber optic trunk line can be up to 80 kilometers, whereas a typical coaxial trunk line is about 10 kilometers, as mentioned above. Prior to the 1990s, cable television systems were not intended to be general-purpose communications mechanisms. Their primary purpose was transmitting a variety of entertainment television signals to subscribers. Thus, they needed to be one-way transmission paths from a central location, known as the head end, to each subscriber's home, delivering essentially the same signals to each subscriber. HFC systems run fiber deep to the cable TV network offering subscribers more neighborhood specific programming by segmenting an existing system into individual serving areas between 500 to 2,000 subscribers. Although networks using exclusively fiber optics would be optimal, presently cable networks equipped with HFC configurations are capable of delivering a variety of high bandwidth, interactive services to homes for significantly lower costs than networks using only fiber optic cables.
FIG. 1 is a block diagram of a two-way hybrid fiber-coaxial (HFC) cable system utilizing a cable modem for data transmission. It shows a head end 102 (essentially a distribution hub) which can typically service about 40,000 subscribers. Head end 102 contains a cable modem termination system (CMTS) 104 that is needed when transmitting and receiving data using cable modems. CMTS 104 is discussed in greater detail with respect to FIG. 2. Head end 102 is connected through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108. Each head end can support normally up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the head end and each distribution node. In addition, because cable modems were not used, the head end of pre-HFC cable systems did not contain a CMTS. Returning to FIG. 1, each of the fiber nodes 108 is connected by a coaxial cable 110 to two-way amplifiers or duplex filters 112 which permit certain frequencies to go in one direction and other frequencies to go in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node 108 can normally service up to 500 subscribers. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along trunk line 116, and subscriber taps, i.e. branch lines 118, make up the coaxial distribution system of an HFC system. Subscribers tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122.
Recently, it has been contemplated that HFC cable systems could be used for two-way transmission of digital data. The data may be Internet data, digital audio, or digital video data, in MPEG format, for example, from one or more external sources 100. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. More notably, they provide up to a thousand times faster transmission of digital data than is presently possible over telephone lines. However, in order for a two-way cable system to provide digital communications, subscribers must be equipped with cable modems, such as cable modem 120. With respect to Internet data, the public telephone network has been used, for the most part, to access the Internet from remote locations. Through telephone lines, data is typically transmitted at speeds ranging from 2,400 to 33,600 bits per second (bps) using commercial (and widely used) data modems for personal computers. Using a two-way HFC system as shown in FIG. 1 with cable modems, data may be transferred at speeds up to 10 million bps. Table 1 is a comparison of transmission times for transmitting a 500 kilobyte image over the Internet.
TABLE 1Time to Transmit a Single 500 Kbytes ImageTelephone Modem (28.8 KBPS)  6-8 minutesISDN Line (64 KBPS)1-1.5 minutesCable Modem (30 Mbps)   1 second
Furthermore, subscribers can be fully connected twenty-four hours a day to service without interfering with cable television service to phone service. The cable modem, an improvement of a conventional PC data modem, provides this high speed connectivity and is, therefore, instrumental in transforming the cable system into a full service provider of video, voice and data telecommunication services.
As mentioned above, the cable industry has been upgrading its coaxial cable systems to HFC systems that utilize fiber optics to connect head ends to fiber notes and, in some instances, to also use them in the trunk lines of the coaxial distribution system. In way of background, optical fiber is constructed from thin strands of glass that carry signals longer distances and faster than either coaxial cable or the twisted pair copper wire used by telephone companies. Fiber optic lines allow signals to be carried much greater distances without the use of amplifiers (item 114 of FIG. 1). Amplifiers decrease a cable system's channel capacity, degrade the signal quality, and are susceptible to high maintenance costs. Thus, distribution system that use fiber optics need fewer amplifiers to maintain better signal quality.
In cable systems, digital data is carried over radio frequency (RF) carrier signals. Cable modems are devices that convert digital data to a modulated RF signal and convert the RF signal back to digital form. The conversion is done at two points: at the subscriber's home by a cable modem and by a CMTS located at the head end. The CMTS converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the operations are reversed. The digital data is fed to the cable modem which converts it to a modulated RF signal (it is helpful to keep in mind that the word “modem” is derived from modulator/demodulator). Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
As mentioned above, cable modem technology is in a unique position to meet the demands of users seeking fast access to information services, the Internet and business applications, and can be used by those interested in cablecommuting (a group of workers working from home or remote sites whose numbers will grow as the cable modem infrastructure becomes increasingly prevalent). Not surprisingly, with the growing interest in receiving data over cable network systems, there has been an increased focus on performance, reliability, and improved maintenance of such systems. In sum, cable companies are in the midst of a transition from their traditional core business of entertainment video programming to a position as full service provider of video, voice and data telecommunication services. Among the elements that have made this transition possible are technologies such as the cable modem.
A problem common to all upstream data transmission on cable systems, i.e. transmission from the cable modem in the home back to the head end, is ingress noise at the head end which lowers the signal-to-noise ratio, also referred to as carrier-to-noise ratio. Ingress noise can result from numerous internal and external sources. Sources of noise internal to the cable system may include cable television network equipment, subscriber terminals (television, VCRs, cable modems, etc.), intermodular signals resulting from corroded cable termini, and core connections. One source of ingress noise is cable modems. In particular, transient noise coming from the upstream transmitter can create noise on the upstream channel. This is described in greater detail below.
The portion of bandwidth reserved for upstream signals is normally in the 5 to 42 MHz range. Some of this frequency band may be allocated for set-top boxes, pay-per-view, and other services provided over the cable system. Thus, a cable modem may only be entitled to some fraction (i.e., a “sub-band”) such as 1.6 MHz, within a frequency range of frequencies referred to as its “allotted hand slice” of the entire upstream frequency band (5 to 42 MHz). This portion of the spectrum—from 5 to 42 MHz—is particularly subject to ingress and transient noise, and other types of interference. Thus, cable systems offering two-way data services must be designed to operate given these conditions.
Although not fully agreed to by all parties in the cable TV and cable modem industry, an emerging standard establishing the protocol for two-way communication of digital data on cable systems has been defined by a consortium of industry groups. The protocol, known as the Multimedia Cable Network System (MCNS), specifies particular standards regarding the transmission of data over cable systems. With regard to the sub-band mentioned above, MCNS specifies that the bandwidth of a data carrier should generally be 200 KHz to 3.2 MHz. Further references to MCNS standards will be made in the specification.
Block 104 of FIG. 1 represents cable modem termination system connected to a fiber node 108 by pairs of optical fibers 106. The primary functions of the CMTS are (1) receiving signals from external sources 100 and converting the format of those signals, e.g., microwave signals to electrical signals suitable for transmission over the cable system; (2) providing appropriate MAC level packet headers (as specified by the MCNS standard discussed below) for data received by the cable system, (3) modulating and demodulating the data to and from the cable system, and (4) converting the electrical signal in the CMTS to an optical signal for transmission over the optical lines to the fiber nodes.
FIG. 2 is a block diagram showing the basic components of a cable modem termination system (item 104 of FIG. 1). Data Network Interface 202 is an interface component between an external data source and the cable system. External data sources (item 100 of FIG. 1) transmit data to data network interface 202 via optical fiber, microwave link, satellite link, or through various other media. A Media Access Control block (MAC) 204 receives data packets from a Data Network Interface 202. Its primary purpose is to encapsulate a MAC header according to the MCNS standard containing an address of a cable modem to the data packets. MAC Block 204 contains the necessary logic to encapsulate data with the appropriate MAC addresses of the cable modems on the system. Each cable modem on the system has its own MAC address. Whenever a new cable modem is installed, its address must be registered with MAC Block 204. The MAC address is necessary to distinguish data from the cable modems since all the modems share a common upstream path, and so that the system knows where to send data. Thus, data packets, regardless of format, must be mapped to a particular MAC address.
MAC Block 204 also provides ranging information addressed to each cable modem on its system. The ranging information can be either timing information or power information. MAC Block 204 transmits data via a one-way communication medium to a Downstream Modulator and Transmitter 206. Downstream modulator and transmitter 206 takes the packet structure and puts it on the downstream carrier. It translates the bits in the packet structure to 64 QAM in the downstream (and 16 QAM or quadrature phase shift keying (QPSK) is used on the upstream path). These modulation methods are known in the art and are also specified in the MCNS protocol. It should be noted that optical fibers used in most cable systems today transmit data in one direction (some fiber optic systems can transmit bi-directional optical signals over a single fiber) and coaxial cables can transmit data in two directions. Thus, there is only one coaxial cable leaving the fiber node which is used to send and receive data, whereas there are two optical fiber lines from the fiber node to the downstream and upstream modulators.
Downstream Modulator and Transmitter 206 converts the digital data packets to modulated downstream RF frames, such as MPEG or ATM frames, using quadrature amplitude modulation, e.g. 64 QAM, forward error correcting (FEC) code, and packet interleaving. Converter 208 converts the modulated RF electrical signals to optical signals that can be received and transmitted by a Fiber Node 210. Each Fiber Node 210 can generally service about 500 subscribers. Converter 212 converts optical signals transmitted by Fiber Node 210 to electrical signals that can be processed by an Upstream Demodulator and Receiver 214. This component demodulates the upstream RF signal (in the 5-42 MHz range in the United States) using, for example, 16 QAM or QPSK. It then sends the digital data to MAC 204.
In accordance with the DOCSIS standard, a cable modem must transmit signals within its designated or allocated upstream channel without creating noise spikes, referred to as spurs, anywhere in the rest of the upstream spectrum. That is, a cable modem must be “quiet” outside the channel it has been designated to transmit data. A problem occurs when either the RF amplifiers in a cable modem's upstream transmitter begin to degrade, the modem's processor degrades over time, or the modem is damaged for any other reason. When the upstream transmitter is faulty, noise signals emitted by a cable modem will look splattered, often with one noticeable spur, throughout the upstream channel. This is often referred to as a transmitter being non-linear in that it creates intermodulation. This is in contrast to being linear where the transmitter simply amplifies its input. The noise created by a faulty cable modem can interfere with the upstream transmission of data on other cable modems.
In addition to being noncompliant with DOCSIS, such degradation effectively reduces the frequency available for other cable modems to transmit data on the upstream data path. A group or system of cable modems typically uses time-division multiplexing and frequency-division multiplexing (the width of frequencies can range from 160 kHz to 2.56 MHz). Thus, noise interference occurring in portions of the frequency spectrum not allocated to a system of cable modems in which a faulty cable modem belongs can cause poor transmission for other systems of cable modems.
Therefore, it would be desirable to be able to detect and identify a cable modem creating unintentional noise in the upstream frequency spectrum. It would be desirable to detect faulty or degrading cable modems during normal operation and in close to real time by measuring noise outside an allocated channel to determine whether a particular cable modem is creating noise spurs. Furthermore, it would be desirable to measure unintentional noise created by a cable modem with reduced manual or human intervention, and have the option of conducting noise checks in different modes of operation, such as continuous or on demand.